Radar sensors are used in motor vehicles for locating other vehicles or other obstacles in the surroundings of one's own vehicle, and for measuring their distances and relative speeds, so that, for instance, an automatic distance control (ACC; adaptive cruise control) is made possible.
A radar sensor for this purpose is described in German Patent No. DE 102 07 437, which has several antenna elements situated next to one another, which, in connection with an optical system in the form of a plano-convex lens, generates several slightly divergent radar lobes. By using several radar lobes whose radar echoes are able to be received by the antenna elements and separately evaluated, a certain angular resolution capability of the radar sensor is achieved. Furthermore, in this known radar sensor, the lens is simultaneously developed at a part of its cross section as a prism by which, from each radar beam generated by a single antenna element, once more a beam component is coupled out that is directed slantwise downwards onto the roadway. This beam component, which hits the roadway surface already at a relatively short distance from the vehicle, is reflected from unevennesses in the roadway, and the radar echoes thus received may, for example, be used to measure the speed of the vehicle over the ground, independently of the signals of the wheel rotary speed sensors. By measuring the distance of the point of impingement of this beam component on the roadway surface, it is also possible to monitor the vertical calibration of the radar sensor. Since the radar echo from the roadway surface is present even when there are no vehicles in the locating range of the main beam of the radar sensor, a loss of sight of the radar sensor may also be detected in the light of the absence of these radar echoes, that may be caused, for instance, by snow or ice in front of the lens of the radar sensor.